The overheating enigma

Hello all, I have a 06 Toyota Tacoma 2.7L with 250,000+. It has been a great little truck with no problems until now. Recently it has been over heating & loosing coolant and cannot find the reason why. There seems to be some backpressure pushing coolant into the overflow tank. The thermostat, hoses, belt, radiator cap & fan clutch have all been replace trying to track down the cause.
I took it to a very reputable shop here in town that specializes in radiators and cooling systems. He did a pressure test 3 different times and found no leaks (leaving the system pumped up for over an hour). Ran the truck and preformed a block test, no sign of combustion gases in the system from a blown or leaking head gasket. No white smoke or coolant shooting out the tailpipe. The owner of the shop has been doing this for over 30 years and he cannot figure out what is wrong. Yet every day the truck overheats and I have to pull over and had 1/3 to a 1/2 gallon of coolant to get it back down to temp.
I really need the truck to last me a few more years and need some help!!!

Everything here indicates and leads to a leaking head gasket, regardless of what your shop says. A radiator pressure test will not show this; you need an engine compression test and leak down test done.

I’d go to an engine specialist.

A chemical test for exhaust residue in the radiator should be definitive. Some head gaskets only leak when the engine is fully up to temp and at highway speeds.

Concur, given what you’ve already done, definitely sounds like exhaust gas is blowing into the coolant. That’s the most plausible explanation for the extra unexplained pressure you are observing in the cooling system, the loss of coolant, and the overheating. & it isn’t totally unexpected at 250K. Did the engine ever overheat one time before all this started to happen? I mean for a known cause, and you fixed it? Even 150 K ago? If so that could have weakened a critical spot in the head gasket, and it just now is breaching.

There are other remote possibilities of course. Something blocking the coolant passages, water pump problem, coolant fan speed problems, etc. To avoid having to spend time and money to rule these out, Oldtimer above is spot on, the next tests should be leak down tests comparing all the cylinders, and a chemical or emission sniffer test for hydrocarbons getting in the coolant.

How much coolant are you loosing by the way?